In existing digital cameras, there is a delay between the time when a user presses the button to take a picture and the time that a final image is actually taken. Between these times, the digital camera determines an average scene intensity, and determines appropriate exposure and gain settings based on the average scene intensity. In some digital cameras, at least nine frames are captured to determine the appropriate exposure and gain settings. If an object that a user is interested in imaging is moving, the object may be out of place or gone by the time the camera is ready to take the final image.
In previous digital cameras, all of the pixels in the pixel array were individually processed to determine the average scene intensity. In some digital cameras, it takes 9 clock cycles to process one pixel. For a VGA size array (i.e., 480×640 pixels), it would take several million clock cycles to process a frame. Some optical sensors allow sub-sampling, where only a portion of the pixels in the array are sampled, such as every other pixel, or every other set of two pixels. Even with sub-sampling, it could take several hundred thousand clock cycles to process a frame.
It would be desirable to provide a faster technique for obtaining average scene intensity information, and thereby minimize the amount of time between when a user pushes the button on a digital camera to take a picture, and the time that a final image is captured.